For years, from his senior position in Estonia's Defense Ministry, Herman Simm leaked highly sensitive NATO intelligence and the names of Western spies to Russia's foreign intelligence service. In a classified damage analysis, NATO concludes that the former KGB colonel was one of the "most damaging" spies in the history of the alliance.

Everyone thought Hermann Simm deserved to be honored. It was Monday, Feb. 6, 2006, and he was dressed in his best suit to attend the day's event. He had been invited to Estonia's presidential palace to accept the "Order of the White Star" for his "service to the Estonian nation." It was an ironic choice.

It wasn't the only medal Simm received for his services that year. The other honor was one that he could only see on his computer screen, supposedly so as to not jeopardize his cover. Sergey Jakovlev, his handler with the SVR, Russia's foreign intelligence service, appeared on the screen to show him his medal. Jakovlev was also the one who informed Simm that he had been promoted to the rank of major general for having supplied Moscow with the names of all suspected and known Russians working as spies for NATO. Then-President Vladimir Putin was very impressed, Jakovlev told his best spy.

Four years on, Simm has now reached the late phase of his career. Indeed, in his field -- spying -- it is not uncommon to spend one's old age in a small prison cell. Simms is incarcerated in a functional, post-Soviet building made of reinforced concrete in the Estonian city of Tartu, where he wears a plain prison uniform and seeks comfort in the Bible. Photos depict him as an older, gray-haired man with a sad look in his eyes.

This is the same man whom NATO, in a classified 141-page report, has recognized as the spy who was "most damaging in Alliance history." The report alleges that Simm, as the former head of security at the Estonian Defense Ministry, had access to most of the classified NATO documents his country received after joining the alliance in the spring of 2004. Until his arrest, in September 2008, he is believed to have secretly handed over thousands of those documents to the Russians. Some of these contained highly sensitive information about NATO's secret defense policies, "including installation, maintenance, procurement and use of cryptographic systems."

28 NATO Countries Sharing Secrets

According to the classified NATO report, the master spy also "compromised a wide range of NATO intelligence reports and analyses," including ones related to fighting terrorism, secret military plans and counterespionage. Never before, the NATO analysis concludes, has a spy betrayed such a large volume of military secrets for such a long time.

Of course, Simm was not the only spy in NATO's past. For years, Rainer Rupp, a West German who went by the codename of "Topaz," supplied classified information to the Stasi, the secret police of the former East Germany. French officer Pierre-Henri Bunel supplied Yugoslavia with NATO bombing plans during the Kosovo crisis. And Daniel James, who was working as a British general's personal interpreter, relayed sensitive details of his country's military operations in Afghanistan to Iran.

Still, the Simm case reveals just how much of a risk the alliance was taking when it gradually expanded eastward after the end of the Cold War. Each of its current 28 member states now enjoys access to almost all the classified information within the alliance. For experts, this is already unsettling enough. But even more worrisome is the fact that members of the old elite -- whose loyalties once lay with a completely different political system -- now work in the security apparatus of some of the new member states. In other words, people like Herman Simm.

A Swift Rise to Power

Simm was born out of wedlock in May 1947 in the small Estonian city of Suure-Jaani. When he was two years old, his mother barely escaped Stalin's ethnic-cleansing operations and deportation to Siberia. Soon thereafter, she married and left the boy to live with his grandmother and aunt. In school, he was considered ambitious, hardworking and well-adapted.

In 1966, when Simm was studying chemistry in Tallinn, he witnessed a brawl between a gang of youths and the police in front of a cinema on the city's outskirts. He intervened and, with his help, the police managed to overpower the gang. The officers were surprised that a student, of all people, had come to their aid. So, they offered him a job. "It was the beginning of his career with the Soviet intelligence service, the KGB," says journalist Mihkel Kärmas, who filmed "The Spy Inside," a documentary on the Simm case for Estonian television.

Simm kept his new job a secret from his family. The aunt, who had been persecuted under Stalin, was appalled when she found out. A short time later, her nephew was confirmed at the church in Suuri-Jaani, which he also kept from his family.

Simm quickly carved out a career for himself with the police. In 1975, he graduated with honors from the Soviet Union's Interior Ministry Academy. Likewise, he joined the Communist Party -- a necessary step, given the fact that his job involved accompanying delegations abroad and that such jobs were reserved for those considered politically reliable. His daughter was born in 1974, the result of an affair with a flight attendant. Today, she works as a computer specialist with Europol, the European police authority.